Electrophotographic printing apparatus having scavengeless development

Abstract: A donor roll (40) for the conveyance of toner in a development system for an electrophotographic printer includes an outer surface (42) of ceramic. The ceramic has a suitable conductivity to facilitate a discharge time constant thereon of less than 600 microseconds. The donor roll (40) is used in conjunction with an electrode structure (41) as used in scavengeless development.

Description: The present invention relates to an electrophotographic printing apparatus with scavengeless development. US-A-3,950,089 discloses a development apparatus in which a surface for the direct conveyance of electrically-conductive toner comprises a dielectric sheath of a thickness of 1-25 mils, having a resistivity of 10 7 to 10 9 ohm-cm. US-A-4,034,709 discloses a development apparatus in which a surface for the direct conveyance of toner comprises styrene-butadiene, of a resistivity of 10 2 to 10 6 ohm-cm. US-A-4,774,541 discloses a development apparatus in which a surface for the direct conveyance of toner is doped with carbon black to a conductivity of 10 -6 to 10 -10 1/ohm-cm. US-A-5,245,392, discloses a phenolic resin coated on a donor roll. The use of phenolic resin coated donor rolls results in discharge time constants less than 300 microseconds. In the prior art, there are a few instances in which the physical properties of ceramics are exploited for various purposes relating to development of electrostatic latent images. US-A-4,544,828 discloses a heating device utilizing ceramic particles as a heat source and adapted for use as a fixing apparatus. US-A-4,893,151 discloses a single component image developing apparatus including a developing roller coated with a Chemical Vapor Deposition ceramic and an elastic blade coated with a ceramic, but not in a scavengeless development system. US-A-5,043,768 discloses a rotating release liquid applying device for a fuser including an outer porous ceramic material. According to the present invention there is provided a donor roll in accordance with appended claim 1. The present invention also provides an electrophotographic printing apparatus in accordance with appended claims 2 to 8. There is provided an apparatus for developing an electrostatic latent image. A housing defines a chamber for storing a supply of toner particles therein. A donor roll, with a ceramic outer coating, is mounted at least partially in the chamber of the housing to advance toner particles to the latent image. An electrode member is positioned in the space between the latent image and the donor roll, closely spaced from the ceramic coating of the donor roll and electrically biased to detach toner particles therefrom so as to form a toner powder cloud in the space between the electrode member and the latent image with detached toner particles from the toner cloud developing the latent image. There is provided an electrophotographic printing machine of the type having an electrostatic latent image recorded on a photoconductive member and a developer unit adapted to develop the latent image with developer material. The improved developer unit comprises a housing defining a chamber for storing a supply of developer material therein. The developer unit also comprises a donor roll, including a ceramic outer coating with a thickness ranging from about 0.17 to about 3.18 mm The donor roll is mounted at least partially in the chamber of the housing and is adapted to advance developer material to the latent image. An electrode member is positioned in the space between the latent image and the ceramic outer coating of the donor roll. The electrode member is closely spaced from the donor roll and is electrically biased to detach developer material from the ceramic outer coating of the donor roll so as to form a powder cloud of developer material in the space between the electrode member and the latent image with detached developer material from the cloud of developer material developing the latent image. There is further provided an electrophotographic printing machine of the type which has an electrostatic latent image recorded on a photoconductive member and a two component developer unit adapted to develop the latent image with developer material. The improved developer unit includes a housing which defines a chamber for storing a supply of carrier granules having toner particles adhering triboelectrically thereto. The improved developer unit also comprises a transport roll mounted in the chamber of the housing for advancing carrier granules and toner particles therefrom. The improved developer unit further comprises a donor roll which includes a ceramic outer coating. The donor roll is mounted at least partially in the chamber of the housing adjacent the transport roll to receive toner particles therefrom and is adapted to advance toner particles to the latent image. An electrode member is positioned in the space between the latent image and the ceramic outer coating of the donor roll. The electrode member is closely spaced from the ceramic outer coating of the donor roll and is electrically biased to detach toner particles from the donor roll so as to form a toner powder cloud in the space between the electrode member and the latent image with detached toner particles from the toner cloud developing the latent image. The present invention will be described further, by way of example, with reference to the accompanying drawing illustrating an elevational view of a developer unit using two component developer material incorporating the features of the present invention therein; Referring to Figure 1, there is shown a development system 38 in some detail. Housing 44 defines a chamber for storing a supply of developer material 47 therein. The developer material includes carrier granules having toner particles adhering triboelectrically thereto. Positioned in the bottom of housing 44 is a horizontal auger 45 which distributes developer material uniformly along the length of transport roll 46 in the chamber of housing 44. Transport roll 46 comprises a stationary multi-pole magnet 48 having a closely spaced sleeve 50 of non-magnetic material, preferably aluminum, designed to be rotated about the magnetic core 48 in a direction indicated by the arrow. Because the developer material includes magnetic carrier granules, the effect of the sleeve rotating through stationary magnetic fields is to cause developer material to be attracted to the exterior of the sleeve. A doctor blade 62 meters the quantity of developer adhering to sleeve 50 as it rotates to the loading zone, the nip 68 between transport roll 46 and donor roll 40. The donor roll is kept at a specific voltage, by a DC power supply 76, to attract a layer of toner particles from transport roll 46 to donor roll 40 in the loading zone. Either the whole of the donor roll 40, or at least a peripheral layer thereof, is preferably of material which has low electrical conductivity, as will be explained in detail below. The material must be sufficiently conductive to prevent any build-up of electric charge with time, and yet its conductivity must be sufficiently low to form a blocking layer to prevent shorting or arcing of the magnetic brush to the donor roll. Transport roll 46 is biased by both a DC voltage source 78 and an AC voltage source 80. The effect of the DC electrical field is to enhance the attraction of developer material to sleeve 50. It is believed that the effect of the AC electrical field applied along the transport roll in nip 68 is to loosen the toner particles from their adhesive and triboelectric bonds to the carrier particles. AC voltage source 80 can be applied either to the transport roll as shown in Figure 1, or directly to the donor roll in series with supply 76. It has been found that a value of up to 200 V rms is sufficient for the output of source 80 for the desired level of reload efficiency of toner particles to be achieved. The actual value can be adjusted empirically: in theory it could be any value up to a voltage of about 400 V rms . The source should be at a frequency of about 2 kHz. If the frequency is too low, e.g. less than 200 Hz, banding will appear on the copies. If the frequency is too high, e.g. more than 15 kHz, the system would probably work but the electronics may become expensive because of capacitive loading losses. Electrode wires 41 are disposed in the space between the belt 10 and donor roller 40. A pair of electrode wires are shown extending in a direction substantially parallel to the longitudinal axis of the donor roll 40. The electrode wires are made from one or more thin (i.e. 50 to 100 µm diameter) stainless steel wires which are closely spaced from donor roller 40. The distance between the wires and the donor roll 40 is approximately 25 µm or the thickness of the toner layer formed on the donor roll 40. The wires are self-spaced from the donor roller by the thickness of the toner on the donor roller. To this end the extremities of the wires supported by the tops of end bearing blocks also support the donor roller for rotation. The wire extremities are attached so that they are slightly below a tangent to the surface, including toner layer, of the donor structure. Mounting the wires in such a manner makes them insensitive to roll runout due to their self-spacing. An alternating electrical bias is applied to the electrode wires by an AC voltage source (not shown). The applied AC establishes an alternating electrostatic field between the wires and the donor roller which is effective in detaching toner from the surface of the donor roller and forming a toner cloud about the wires, the height of the cloud being such as not to be substantially in contact with the belt 10. At the development zone, i.e., the region where the photoconductive belt 10 passes closest to donor roll 40, a stationary shoe 82 bears on the inner surface of the belt. The position of the shoe relative to the donor roll establishes the spacing between the donor roll and the belt. The position of the shoe is adjustable and it is positioned so that the spacing between the donor roll and photoconductive belt is preferably about 0.4 mm. Another factor which has been found to be of importance is the speed with which the sleeve 50 is rotated relative to the speed of rotation of donor roll 40. In practice both would be driven by the same motor, but a gear train would be included in the drive system so that sleeve 50 is driven at a significantly faster surface velocity than is donor roll 40. A transport donor roll speed ratio of 3:1 has been found to be particularly advantageous, and even higher relative speeds might be used in some embodiments of the invention. In other embodiments the speed ratio may be as low as 2:1. According to the present invention, and referring to Figure 1, on the outer surface of donor roll 40 is a ceramic coating 42. A ceramic coating is a non-metallic, inorganic compound normally comprised of a blend pure oxide ceramics such as alumina, zirconia, thoria, beryllia, magnesia, spinel, silica, titania, and forsterite, which may be applied as a film to a metal substrate. Ceramics which include at least one of aluminum (Al), boron (B), carbon (C), germanium (Ge), silicon (Si), titanium (Ti), zirconium (Zr), magnesium (Mg), beryllium (Be) and tungsten (W) are particularly hard, highly abrasion resistive, have high resistivity, high dielectric strength, low dielectric loss, and a high dielectric constant and are, therefore, preferred for donor roll coating. When this outer roll of ceramic is used, the core of donor roll 40 is intended to be of a conventional conductive material, such as aluminum. This ceramic coating is preferably plasma sprayed onto the core of the donor roll 40 with material properties and thicknesses chosen to obtain a preselected conductivity, and, if necessary, ground down through techniques well-known in the art to assume the desired precise dimensions for a particular development apparatus. The wall thickness of the ceramic coating 42 is between 0.17 and 3.18 mm, on a donor roll 40 having a total outer diameter of approximately 25 mm; this thickness represents a compromise between concerns of ceramic material cost and grinding cost. It has been found that this ceramic coating is particularly suited for the design parameters of a donor roll in scavengeless development, either of the magnetic brush or single-component variety. Because the ceramic coating may be made with relatively thick walls, the thickness of the walls can be exploited to ensure that surface anomalies such as craters or pin holes are kept to a minimum. The use of a plasma spray method of applying the ceramic coating results in a much more uniform periphery geometry than that obtained from phenolic resin coating. Thus, grinding subsequent to plasma coating can often be eliminated. And, once again, because the ceramic coating is relatively easily worked, it is possible, if necessary, to grind down such a cylinder to a small extent to ensure precise dimensions. The use of ceramic coated donor rolls results in discharge time constants roughly of from about 600 microseconds to slightly less than 60 microseconds. Discharge times as low as 60 microseconds greatly reduce discharge time and improve copying speed over similar systems with anodized aluminum donor rolls. Ceramic coating has been shown to be a suitably hard substance which has presented no significant abrasion problems when placed within moving contact with a magnetic brush for an extended period. Many suitable compositions of ceramics have hardnesses in excess of Rockwell "C" 60 and are much harder than phenolic coatings. Thus, the use of harder ceramic materials results in fewer scratches and corresponding improvements in image copy quality.

Claims: An electrophotographic printing apparatus of the type having an electrostatic latent image recorded on a photoconductive member and a developer unit adapted to develop the latent image with toner particles, wherein the developer unit comprises: a housing (44) defining a chamber for storing a supply of toner particles (47) therein; a donor roll (40) including a ceramic outer coating (42), said donor roll (40) being mounted at least partially in the chamber of said housing and being adapted to advance toner particles to the latent image; characterized in that said apparatus includes an electrode member (41) positioned in the space between the latent image and said ceramic outer coating (42) of said donor roll (40), said electrode member (41) being closely spaced from said ceramic outer coating (42) of said donor roll (40) and being electrically biased to detach toner particles from said ceramic outer coating (42) of said donor roll (40) so as to form a toner powder cloud in the space between said electrode member (41) and the latent image with detached toner particles from the toner cloud developing the latent image, wherein the ceramic outer coating of said donor roll (40) has a discharge time constant sufficient to form the toner powder cloud. An apparatus a claimed in claim 1, wherein the ceramic outer coating (42) has a thickness ranging from about 0.17 to about 3.18 mm. An apparatus as in claim 1 or claim 2, wherein said ceramic outer coating (42) of said donor roll (40) has a discharge time constant less than 600 microseconds. An apparatus as claimed in any of claims 1 to 3, wherein said ceramic outer coating (42) of said donor roll has a conductivity greater than 10 -8 (ohm-cm) -1 . An apparatus as claimed in any one of claims 1 to 4, wherein said electrode member (41) includes a plurality of wires spaced from one another. An apparatus as claimed in any one of claims 1 to 5, further comprising a transport roll (46) mounted in the chamber of said housing (44) and being positioned adjacent said ceramic outer coating (42) of said donor roll (40), said transport roll (46) being adapted to advance toner particles to said ceramic outer coating (42) of said donor roll (40). An apparatus as claimed in claim 6, further comprising means for applying an alternating electric field between said donor roll (40) and said transport roll (46) to assist in transferring at least a portion of the toner particles from said transport roll to said ceramic outer coating of said donor roll. An apparatus as claimed in claim 7, wherein said applying means applies an electrical field that alternates at a selected frequency ranging between about 200 Hz and about 20 kHz with a voltage less than 400 V rms .

Inventors: BEHE THOMAS J, GILMORE III DANIEL R, BEHE, THOMAS J., GILMORE III, DANIEL R.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1994
Main classification: G03G15
IPC codes: G03G15